Method of applying the analysis of scrub mark morphology and location to the evaluation and correction of semiconductor testing, analysis, and manufacture

ABSTRACT

By examining scrub mark properties (such as position and size) directly, the performance of a wafer probing process may be evaluated. Scrub mark images are captured, image data measured, and detailed information about the process is extracted through analysis. The information may then be used to troubleshoot, improve, and monitor the probing process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/046,559, filed Oct. 27, 2001 now abandonment, which claims benefit of60/244,432, filed Oct. 30, 2000, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

Aspects of the present invention relate generally to semiconductortesting equipment, and more specifically, to methods of analyzing scrubmarks.

BACKGROUND

A variety of equipment and techniques have been developed to assistmanufacturers of integrated circuits for testing those circuits whilestill in the form of dies on semiconductor wafers. In order to quicklyand selectively electrically interconnect metalized contact pads (alsoknown as “bonding pads”) on each die to electrical test equipment (knownas a “prober machine”), arrays of slender wires or other contact mediaare provided. The contact media are arranged on conventional printedcircuit boards so as to be positionable on the metalized contact padsassociated with each semiconductor die. As is well known by those ofordinary skill in the art, those printed circuit board test cards havecome to be known as “probe cards” or “probe array cards”, and thecontact media have come to be known as “probe card pins” or “probe pins”or “probe wires”.

As the component density of semiconductor circuits has increased, thenumber of contact pads associated with each die has increased. It is nownot uncommon for a single die to have upwards of 600 pads electricallyassociated with each die. The metalized pads themselves may have aslittle as a ten μm gap there between with an on-center spacing on theorder of 50 μm to 100 μm. As a result, the slender probe wires of theprobe array cards have become much more densely packed. It is highlydesirable that the free ends or “tips” of the probes be aligned in acommon horizontal plane, as well as have the proper positioning withrespect to one another within the plane so that when the probes arepressed down onto the metalized pads of an integrated circuit die by aprober machine, the probes touch down substantially simultaneously, andwith equal force while being on target. As used herein, the terms“touchdown”, “rest” and “first contact” have the same meaning. In theprocess of making electrical contact with the pads, the probes are “overtraveled” causing the probes to deflect from their rest position. Thismovement is termed “scrub” and must be taken into account in determiningwhether the rest position and the over travel position of the probes arewithin specification for the probe card.

The assignee of the present invention has developed equipment fortesting the electrical characteristics, planarity and horizontalalignment, as well as scrub characteristics of various probe cards andsells such equipment under its Precision Point™ line of probe card arraytesting and rework stations. A significant component of these stationsis a planar working surface known as a “check plate”. A check platesimulates the semiconductor die undergoing a test by a probe card whilechecking the above described characteristics of the probes. A suitablecheck plate for use with the assignee's Precision Point™ equipment isdescribed in detail in U.S. Pat. No. 4,918,374 to Stewart et al. issuedApr. 17, 1990, the disclosure of which is incorporated herein byreference. It is sufficient for the purposes of this disclosure toreiterate that while the subject probe card is held in a fixed positionthe check plate is moved horizontally in steps when testing thehorizontal relative positioning, and vertically in steps when testingthe touchdown contact and over travel position of each probe tip.Previously, and as described in the above-identified patent, horizontalposition information for each probe tip was determined by translating anisolated probe tip in steps across resistive discontinuities on thecheck plate. In recent years, this technique has been altered by placinga transparent, optical window in the surface contact plane of the checkplate with a sufficiently large surface dimension so as to permit aprobe tip to reside thereon. An electronic camera viewing the probe tipthrough the window digitizes the initial touch down image of the probe,and a displaced position of the probes due to “scrub” as the check plateis raised to “over travel” the probe. The initial touch down position iscompared to the anticipated touch down position to assist an operator inrealigning that particular probe.

Another prior art technique for determining relative probe tip positionsin a horizontal (e.g. X-Y) plane is described in U.S. Pat. No. 5,657,394to Schwartz et al., the disclosure of which is incorporated herein byreference. The system disclosed therein employs a precision movementstage for positioning a video camera into a known position for viewingprobe points through an optical window. Analysis of the video image andthe stage position information are used to determine the relativepositions of the probe points. In systems of this type, a “reference”probe position is determined primarily through information from thevideo camera, combined with position information from the precisionstage. If the pitch of the probes on the probe card is small enough, twoor more probes can be simultaneously imaged with the video camera. Theposition of this adjacent probe is then referenced with respect to the“reference” probe from information from the video camera only. Thecamera is then moved to a third probe, adjacent to the second probe andthis process is repeated until each probe on the entire probe card hasbeen imaged.

In addition to the above devices for measuring various parameters ofprobe cards, equipment is available for measuring actual “scrub marks”made by probe card pins on a test wafer which has been impressed by theprobe card with a prober machine. One such apparatus is manufactured byVisioneering Research Laboratory, Inc., Las Cruces, N.Mex. to providehigh quality imaging of scrub marks made by a probe card and a probermachine. It is well known that scrub patterns analyzed by a probe cardanalysis machine do not match the scrub marks produced on a test waferimaged by a scrub mark analysis machine. The test wafer models thesurface characteristics of bonding pads on a semiconductor die. Asstated above, the measurement surface on the probe 15 card analyzer istypically manufactured from hardened steel, or more recently atransparent synthetic or natural crystal such as sapphire. This probecard analysis testing surface is much harder than the aluminized surfaceof a semiconductor bonding pad. The typical annealed aluminum surface ofa semiconductor bonding pad in fact yields under pressures applied bythe semiconductor probing machine which may be on the order of 5 gramsper pin. Remembering that the pin surface is very small, the pressureapplied is sufficient to break the surface of the aluminum bonding padcausing the probe tip to ‘dig in’ during probe pin overtravel. Within ashort distance, the tip of the probe pin plows so deeply into thealuminum surface that it stops even though the probe card continues itsdownward travel. This phenomenon has been characterized as “stubbing” bythe assignee of the present invention. In contrast, the hard metal orsapphire surface of the probe card analysis machine does not yield underpressure from the probe pin. In addition, the metal or sapphire contactsurface of the probe card analysis machine is highly polished and has amuch lower coefficient of friction than does the aluminized surface ofthe semiconductor die bonding pad.

As a result, the probe pin does not stub on the probe card analysismachine, and the probe pin tip travels further than it does on thealuminized bonding pad. Furthermore, the place at which the probe pinfirst contacts an aluminized bonding pad (or the aluminizedsemiconductor test wafer which simulates the bonding pad in the scrubmark analysis machine) or “touch downs” position of the probe pin is notreadily discernable in the scrub mark made in the aluminum surface. Thescrub mark resembles a brush stroke with a faint starting position and adeep, clearly defined ending position. Conversely, the probe cardanalysis machine accurately captures the touch down position of theprobe pin on the measuring surface as well as its full travel across thesurface without stubbing. Therefore, neither the touch down position,nor the end of travel position of the probe pin on the probe cardanalysis machine, matches corresponding positions on either an actualaluminum bonding pad or on a semiconductor test wafer imaged by a scrubmark analysis machine.

SUMMARY

By examining scrub mark properties (such as position and size) directly,the performance of a wafer probing process may be evaluated. Scrub markimages are captured, image data measured, and detailed information aboutthe process is extracted through analysis. The information may then beused to troubleshoot, improve, and monitor the probing process.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of a semiconductor wafer having aplurality of dies.

FIG. 2 is a enlarged, schematic representation of a semiconductor dieindicated at circled area 2 of FIG. 1.

FIG. 3 is a sectional, schematic side elevation view of a probe cardhaving a plurality of probe pins, and of a semiconductor wafer.

FIG. 4 is a schematic representation of a scrub mark analyzed by a scrubmark analysis machine, and a scrub pattern analyzed by a probe cardanalysis machine.

FIG. 5 (a through h) is a schematic representation of semiconductor diebonding pads having scrub marks thereon due to various probe card and/orprober machine errors.

FIG. 6 is a schematic representation of one method of combining datasets.

FIG. 7 is a block schematic representation of system for applyinganalyses of scrub mark morphology and location.

DETAILED DESCRIPTION

A semiconductor wafer is generally indicated at reference numeral 10 inFIG. 1. A semiconductor wafer has a plurality of dies 12 arranged inorthogonal rows and columns across the surface of the wafer. Aconventional wafer may have a diameter of up to eight inches andanywhere from 200 up to thousands of dies per wafer depending upon thecomplexity of the semiconductor circuits imbedded in each die. Arepresentative die is generally indicated at reference number 12 in FIG.2. The die has a plurality of bonding pads 14 and a plurality ofsemiconductor circuits 16 surrounded by the bonding pads. The bondingpads typically have an aluminized surface which has been annealed. Thebonding pads serve as an electrical intermediary between the worldoutside of the integrated circuit 16 and the circuit itself. Small wires(not shown) interconnect the bonding pads 14 with the semiconductorcircuit 16. Another set of small wires (also not shown) interconnect thebonding pads with external pins (not shown) in a ceramic, ordual-in-line package (DIP) for connecting the integrated circuit into alarger circuit.

As shown in FIG. 3, the bonding pads also provide positions forelectronic probe pins 18 on a semiconductor probe card 20 to contact thebonding pads. As is well known to those of ordinary skill in the art,the semiconductor probe card 20 is received in a prober machine (notshown) as well as is the semiconductor wafer 10 so that when the probepins 18 are in contact with the bonding pads 14 a plurality ofelectronic tests can be performed on the semiconductor circuit 16.

As is also well known to those of ordinary skill in the art, it iscommon for the probe card pins 18 to become misaligned during use. Oncethe misalignment has exceeded predetermined tolerances, the probe cardmust be reworked and/or remanufactured to bring the tips 22 of the probepins back into planarity, as well as back into horizontal registrationwith respect to one another, and with respect to the probe card 20. Forthis purpose, a variety of machines known as probe card analyzers 70have been constructed in which a relatively hard testing surface oftentermed a “check plate” is positioned in place of the semiconductor wafer10. The check plate may have a very hard transparent window manufacturedfrom sapphire or another synthetic crystal, or may be constructed from atool hardened steel having various electrical discontinuities thereon sothat the horizontal positioning of the respective pin tips 22 may bemeasured with respect to one another and with respect to the probe card.It is common during such testing that probe pins are over-traveled inthe vertical (i.e. “Z”) direction and will “scrub” along the surface ofthe check plate. As best seen in dashed lines in FIG. 4 at referencenumber 24, a probe card analysis scrub pattern has a well definedstarting or “touch down” point 26 and a well defined end of travel point28. Unfortunately, due to the hardness of the check plate surface, thisscrub pattern does not match a corresponding scrub mark 30 shown insolid lines in FIG. 4 made in an actual bonding pad or test wafer. Scrubmark 30 can be electronically imaged 72 by an appropriate scrub markanalysis machine 74. The starting or touch down point 32 of the scrubmark in an aluminized surface such as a semiconductor die bonding pad isdifficult to discern because the probe pin 18 is applying relativelylittle force to the aluminized surface. In fact, the touch down point onthe aluminized surface should be where the touch down point 26 is shownwhen the same probe touches down on a probe card analysis check plate.However, the probe “skates” along the surface of the aluminized bondingpad before it begins to make a discernable mark, and the distancebetween the touch down point 26 as measured by the probe card analysismachine 70, and the touch down point 32 as measured by the scrub markanalysis machine 74 has been termed by the Applicant as “skatingdistance” 34.

Similarly, the end of travel 36 of the probe tip in either an aluminizedbonding pad, or the surface of a test wafer in a scrub mark analysismachine 74 falls short of the end of travel point 28 as indicated by theprobe card analysis machine 70. This is because shortly after the probetouches down at touch down point 32, the probe tip digs into thealuminized surface of the bonding pad as a plow enters the ground. Thecoefficient of friction between the probe pin tip and the bonding padquickly rises. As a result thereof, the probe “stubs” into the metalizedsurface when the force due to friction equals the forward force appliedby the prober machine through the probe pin. The distance between theend of travel point 36 as measured by the scrub mark analysis machine 74and the end of travel point 28 measured by the probe card analysismachine 70 has been defined by the Applicant as the “stubbing” distance38.

It is apparent that in predicting the behavior of a probe card pin on asemiconductor bonding pad, it is the probe card analysis machine touchdown point 26 and scrub mark analysis machine end of travel point 36which are of principal interest to the operators of semiconductor probermachines. That is, it is undesirable to have the probe card pin touchdown outside of the bonding pad area onto the soft passivation layer ofthe semiconductor die (and also in violation of various militarystandards for semiconductor products). It is also undesirable to havethe probe pin 18 severely deformed by excessive stubbing represented bystubbing distance 38 so as to put either excessive pressure on thebonding pad such as to damage the pad or bend the pin. In addition, itis highly desirable, as shown in FIG. 5, to determine the source oferrors in a probe card and prober machine combination which may be dueto errors in the prober machine itself.

FIG. 5 schematically illustrates a series of bonding pads on asemiconductor die having scrub marks left by a probe card/ probercombination. As shown in FIG. 5( a), all of the scrub marks aresubstantially centered in the pads as is desired. FIG. 5( b) illustratesthat either the probe pins, or more likely the prober machine itself,has offset the pins in the negative X direction. FIG. 5( c) illustratesthe situation in which the prober machine probably has an offset in thepositive Y direction. FIG. 5( d) illustrates that the probe card hasbeen rotated about the Z-axis in a clockwise direction, or the pins havebeen twisted in that direction. FIG. 5( e) illustrates excessively longscrub marks in both the X and Y directions, indicating that the proberis probably exerting too much force on the probe pins. Conversely, FIG.5( f) has small scrub marks which are not elongated, indicating thatinsufficient pressure is being applied in the Z direction by the probermachine. FIG. 5( g) shows elongated scrub marks on the left hand side ofthe die, and very short scrub marks on the right hand side of the die.This configuration indicates a pitch error about the Y axis. FIG. 5( h)indicates a roll error about the X-axis such that too much pressure isexerted on the probe pins in the upper portion of the die, and toolittle pressure is exerted on the pins in the lower portion of the die.

By combining scrub mark analysis data from the scrub 10 mark analyzer 74and scrub pattern data from the probe card analysis machine 70, it ispossible to predict more accurately the behavior of a probe pin on asemiconductor die metalization pad, as well as isolate whether thesource of errors in probe pin position is due to the prober machine, orthe position of the probe pins with respect to the probe card itself.

In a first embodiment of the invention, correction factors in the X, Y,and θ directions (left-right, up-down, and clockwise counterclockwise asshown in FIG. 6) are calculated so as to minimize differences in datasets relating to the stubbing distance 38 shown in FIG. 4. FIG. 6illustrates a representative set of orthogonal bonding pads 14 on asemiconductor die surface 12 in the X and Y directions. The pads havescrub marks 30 such as are to be analyzed by a scrub mark analysismachine 74. In an iterative fashion, a correction factor in the X, Y andθ directions is added to a combined data set stored in a computer by aconventional computer program, such as the Excel database programavailable from Microsoft Corporation, Redmond, Washington. An errorvalue is then associated with the stubbing distance 38 measured for eachprobe pin 18 associated with a bonding pad 14. As the X, Y and θcorrection factors or “offset values” are incremented, a minimum errorvalue represented by the stubbing distance 38 will be found. Thatminimum error value and the corresponding X, Y and θ offsetsquantitatively represent the degree to which the probe card and probermachine combination are out of tolerance. The error value may becalculated by simple summation of the stubbing distance 38, as describedabove by averaging all of the stubbing distances for all of the pads; orby summing the squares of the differences between the average stubbingvalues and the differences between the end of travel positions betweenthe probe card analysis machine 70 and scrub mark analysis machine 74;or, a standard deviation of the ending position differences between thescrub mark analysis and probe mark analysis machines. The specificnumerical method which the artisan of ordinary skill uses is aninconsequential activity with respect to the claimed invention.

Alternatively, in a second embodiment of the invention, the error valuemay be associated with the skating distance 34 shown in FIG. 4. In thisalternate embodiment of the invention, an error value may be assigned tothe sum of the differences between the average skating distance 34 forall of the scrub marks 30 and the sum of the differences between thetouch down points 26 and 32 as measured by the probe card analysismachine 70 and scrub mark analysis machine 74, respectively. As statedwith respect to the first embodiment of the invention, a sum of thedifference of the squares, or a standard deviation technique may also beused to define the error value. As stated with respect to the firstembodiment, in this alternate embodiment correction factors in the Xdirection, Y direction and θ direction are incrementally applied to themeasured locations of the touch down points correlating to the skatingdistance 34 until the error value is minimized. The correction factorsin X, Y and θ thus relate to the degree to which the probe card andprober machine combination are out of tolerance.

In a third embodiment of the invention, the center of the scrub marks isdefined by the scrub mark analysis machine 74 mathematically as astraight line between the touch down point 32 and end of travel point 36measured by the scrub mark analysis machine 74 and a mathematicalstraight line between the touch down point 26 and end of travel point 28as defined and measured by the probe card analysis machine 70. Thisscrub mark center line 40 can also be directly measured by the scrubmark analysis machine 74 from the left or right edge of the bonding pad14 in the X-axis and the top or bottom edge of the bottom edge in theY-axis as shown in FIG. 6. The error value can then be assigned as thesum of the absolute values of the center lines 40 with respect to theedge of their respective bonding pad or as the sum of the squares ofthose measurements. By incrementing correcting factors in the X, Y and θdirections, the minimum error value resulting from that incrementalanalysis gives the X, Y and θ correction factors which arerepresentative to the degree to which the probe card and prober machinecombination are out of tolerance.

In a fourth embodiment of the invention, the minor axis 42 of the scrubmarks can be measured by both the scrub mark analysis machine 74 and theprobe mark analysis machine 70 and the error factors applied as setforth above with respect to the third embodiment of the invention.

Furthermore, by comparing the data sets collected for the scrub patternsfrom the probe card analysis machine 70, the scrub marks from the scrubmark analysis machine 74, and the orientation of the errors as shown inFIG. 5, it can be determined whether the errors are due to misalignmentof the probes with respect to the card, or the card with respect to theprober machine. These errors are particularly apparent where the diesbeing tested are located at the perimeter of the semiconductor wafer.

Finally, it is desirable to scrub the test wafer 10 in the probe machinewith a hot chuck or other means for heating the wafer and the probe card20 to the approximate operating temperature of an integrated circuit ofa number of embodiments of the invention have been described.

Probe card data is generated by removing all prober errors from thescrub mark data set. Within the probe card data set, multiple probe cardparameters are reported, based on the probe's scrub size, position,repeatability, and correlation patterns. Software 76 is used that hasacquisition, staging, image processing, image analysis and informatics(database) components.

For each pad on the wafer, the pattern of scrub marks within each pad isanalyzed and characterized in terms of length, width, orientation, andposition relative to the pad. Information within a single die yieldsinformation about the quality of the probe card. Information betweendies yields information about the prober itself, the wafer manufacturingprocess, and the environmental conditions of the fabrication and testfacilities. In some probers, the probe card has the capacity to testmore than one die at a time. In that case, the plurality of dies isreferred as a dut. The present invention may also test duts as well asindividual dies. In addition, a simple aluminized surface can be probedand the morphology and location of the scrub marks can be used toanalyze the probe card and the prober system in the absence of an actualsemiconductor wafer.

The scrub marks reveal a great deal about the entire manufacturing andtesting process used in the fabrication facilities. By applyingregression and clustering methodologies to the study of patterns ofscrub marks, a number of parameters can be deduced from the images.

It will be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. Accordingly, otherembodiments are within the scope of the following claims.

1. An opto-mechanical workstation for loading and systematically movingwafers and for imaging and analyzing scrub marks; said workstationcomprising: a probe card analyzer; a scrub mark analyzer; an imagingapparatus configured and operative to obtain images of first scrub marksmade by probe card pins on a check plate in said probe card analyzer andimages of second scrub marks made by said probe card pins on bondingpads in said scrub mark analyzer; and a data processor coupled to saidimaging apparatus and configured and operative to obtain scrub mark dataassociated with said first scrub marks and scrub pattern data associatedwith said second scrub marks and to analyze said scrub mark data andsaid scrub pattern data; wherein said data processor allows predictionof the behavior of a probe pin on a semiconductor die metalization pad.